Field of Invention
This patent application relates generally to a method and apparatus for precisely dispensing multiple aliquots of a fluid from a reservoir of fluid or precisely aspirating aliquots of fluid into said reservoir. The fluid in the reservoir may alternatively be manually aspirated and dispensed by the apparatus. The volume of the aliquot can readily be varied. This invention has particular application in laboratory practice for aspirating a quantity of fluid into a serological pipette and then dispensing precise aliquots of the fluid.
Background
Serological pipettes are widely used for liquid measurement and dispensing in laboratories that perform, for example, drug development, environmental testing, and diagnostic testing. These pipettes may be described as glass or plastic straws, and may be, for example, approximately 30 cm long with graduations printed on them. Traditionally, liquid was drawn into these pipettes by applying suction to the top end by mouth or a rubber bulb. Liquid is measured by aspirating to a graduation line, and then dispensed by removing the suction. Current practice often employs a pipette controller such as a Drummond Scientific Pipette-Aid or a BrandTech Scientific acu-j et Pro Pipette Controller which use a small battery powered air pump and trigger-style pneumatic valves to manipulate pressure inside of serological pipettes to draw up and expel liquid.
Frequently, multiple aliquots of a sample must be dispensed for the analytical process. To do this the user first aspirates slightly more than the required volume and then slowly dispenses sample until the meniscus of the fluid aligns with a graduation line on the serological pipette. This is the starting volume. The user must note this reading and then dispense fluid until the meniscus drops to the graduation line corresponding to the difference between the starting volume and the desired dispense volume. If another aliquot is required, the user dispenses again to the graduation line corresponding to the difference between the prior reading and the desired volume. This methodology has many problems. It is time consuming because the meniscus must be carefully read for each dispense. This requires holding the pipette controller very steady while reading the meniscus and simultaneously dispensing into the correct test vessel. This is a time consuming and fatiguing process when it must be repeated many times.
There are also multiple sources of error with the above described method: the meniscus must be read twice to obtain an accurate reading, and the user must subtract the first reading from the second reading. This is easy when a common volume like 1 ml is needed, but difficult for repetitive dispensing of 1.3 ml, for example. There is also an error associated with taking the difference between two larger numbers. For example, one can read a 25 ml serological pipette to an accuracy of 0.25 ml or 1%. However, if one attempts to dispense 25 aliquots of 1 ml this 0.25 ml error translates to a potential error of 0.5 ml since two readings are required. This is an error of 50% which is not acceptable for most analyses.
Previous methods to dispense multiple aliquots of fluid have depended upon methods that are cumbersome and lack flexibility. For example, U.S. Pat. No. 4,406,170 describes a device that can dispense aliquots from a syringe. This device can be quite accurate; however, it requires the use of syringes which are much more expensive than serological pipettes, are much harder to load into the device, do not easily enable the range of volumes, and cannot reach into vessels that require a longer length.
Piston operated, air-displacement pipettes such as one described in U.S. Pat. No. 4,821,586 are capable of dispensing multiple aliquots. However, this method requires a piston displacement that is equal to the volume to be aspirated. Serological pipettes are often used to aspirate 50 ml. This method requires a very large and impractically sized piston to aspirate this large of a volume. In addition, the range of volumes that can be dispensed accurately is limited because of the air contained between the liquid sample and the piston—the “dead volume.” As the dead volume increases, the accuracy decreases. This method therefore requires several sizes of pipettes to accurately dispense the normal volumes used in a laboratory.
U.S. Pat. No. 7,396,512 attempts to overcome the above difficulties by controlling the time that air flows into a serological pipette to control the volume dispensed. Pressures on both sides of the valve are monitored. This design has several fundamental shortcomings. One shortcoming is that the volume dispensed will be decreased if the back pressure from the serological pipette is increased by, for example, the tip of the serological pipette being partially occluded by a vessel wall or if the tip is immersed in fluid. The flow is also dependent upon the viscosity of the liquid dispensed. Another difficulty is that the delivered volume is dependent upon the size of serological pipette attached to the device. This means that the user must inform the device of the size pipette being used. In most labs, serological pipettes are disposable and changed constantly, oftentimes with a different volume capacity. This device requires the user to enter the volume and the manufacturer of the serological pipette to obtain accurate results. This is time consuming and an impractical burden on the user.
Therefore, what is required is a pipette controller that can aspirate fluid into a serological pipette and then quickly and accurately dispense a series of aliquots by simply depressing a button. In addition, the volume of the aliquot can be easily set, and the volume dispensed is not dependent upon the size of serological pipette that is mounted to the pipette controller, the viscosity of the sample, or how the sample is dispensed.